1. Field of the Invention
Embodiments of the invention relate to the field of semiconductor device fabrication. More particularly, the present invention relates to a system for controlling ribbon beam uniformity in an ion implanter by adjusting the beam current profile along the beam path.
2. Discussion of Related Art
Ion implantation is a process used to dope impurity ions into a semiconductor substrate to obtain desired device characteristics. An ion-beam is directed from an ion source chamber toward a substrate. The depth of implantation into the substrate is based on the ion implant energy and the mass of the ions generated in the source chamber. One or more ion species may be implanted at different energy and dose levels to obtain desired device structures.
FIG. 1 is a block diagram of an ion implanter 100 including an ion source chamber 102. A power supply 101 supplies the required energy to source 102 which is configured to generate ions of a particular species. The generated ions are extracted from the source through a series of electrodes 104 and formed into a beam 10 which passes through a mass analyzer magnet 106. The mass analyzer is configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio are able to travel through the analyzer for maximum transmission through the mass resolving slit 107.
Ions of the desired species pass from mass slit 107 through a deceleration stage 108 comprising multiple electrodes with defined apertures that allow the ion beam to pass through. By applying different combinations of voltage potentials to the multiple electrodes, the deceleration stage 108 manipulates the ion energies. A corrector magnet 110 shapes or collimates the ion beam generated from the deceleration stage 108 into the correct form for deposition onto a wafer or substrate. In particular, the corrector magnet receives a divergent ion beam and collimates the beam. Corrector magnet 110 is energized to deflect ion beamlets in accordance with the strength and direction of the applied magnetic field to provide a ribbon beam targeted toward a work piece or substrate positioned on support (e.g. platen) 114. In addition, the corrector magnet 110 filters out any ions from the beam that may have been neutralized while traveling through the beamline. In some embodiments, a second deceleration stage 112 may be disposed between corrector magnet 110 and support 114. The ions lose energy when they collide with electrons and nuclei in the substrate and come to rest at a desired depth within the substrate based on the acceleration energy.
Generally, beam current, energy contamination and beam uniformity (current density and angle of implantation) are the parameters that jeopardize device throughput during semiconductor manufacturing processes. For example, if the beam current is too low, this will reduce the throughput of the implanter for a given total ion dose. Energy contamination occurs when there is a small fraction of the ion beam that is at a higher energy than desired which rapidly increases the depth of the desired junction that is formed in the substrate when creating an integrated circuit. This leads to degraded performance of the desired circuit profile.
Beam uniformity is determined, in part, by the profile of the ion beam used for ion implantation. The beam profile is a map of ion beam intensity in a plane orthogonal to the direction of beam transport. The beam current may vary over the cross-sectional area of the ion beam, particularly in the case of large area beams such as ribbon ion beams. Furthermore, the beam profile may vary with implant conditions, such as dopant species, energy and current, and with time. These variations can compromise the desired device characteristics which produce lower manufacturing yields and lead to higher processing costs. Accordingly, it is desirable to measure and, if necessary, adjust the beam profile in order to enhance ion implanter performance before the ribbon beam is incident on the surface of a wafer or substrate.